New photodetectors for high resolution PET

Solid-state photomultipliers (SSPMs) are a novel type of photodetectors that have shown to be promising for PET, especially for small animal scanners with reduced ring diameter and combined PET / MRI systems. SSPMs are compact devices composed of a few thousand of micro avalanche photodiodes operated in Geiger mode. They combine the advantages of photomultipliers tubes and avalanche photodiodes. They have high gain (equal or greater than photomultipliers tubes) when operated at bias voltages under 100 V and do not require preamplification or cooling as APDs do. In addition they are relatively insensitive to magnetic fields which makes them ideal for PET/MRI.

Our goal is to study and optimize the performance of PET detectors composed of finely pixelated crystal arrays coupled to position-sensitive SSPMs, for their integration in small animal PET scanners or MR-compatible PET modules. Our work is focused on the characterization of two different large-area SSPMs with position sensitive capability. Arrays of 4 x 4 SSPMs with individual element size of 1.5 x 1.5 mm² were studied together with continuous 5 x 5 mm² SSPMs.

Fig. 1:Left: 4 x 4 array of 1.5 x 1.5 mm2 SSPMs. Middle: close up of the SSPM array. Right: continuous 5 x 5 mm2 position-sensitive SSPM mounted on a dedicated readout board.

Uniformity of the performance across the area of the device was studied using a light source and we showed a good uniformity of the response and the spatial resolution across the sensitive area of the SSPM.
Preliminary results with SSPMs coupled to crystal arrays showed that both devices could resolve arrays with 0.5 mm x 0.5 mm cross-section LSO crystals. Studies of optimal operating conditions such as temperature or bias voltage were conducted as well and demonstrated that position sensitive SSPMs can be operated at room temperature without preamplifiers. Future work will include studies of the timing properties of those detectors as well as dual-ended readout schemes for depth of interaction encoding applications.

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